A goal of sound reproduction equipment is to provide a life-like sound quality to the listener. Life-like sound quality is understood to be best achieved when a sound system including the speakers have a flat frequency response curve throughout the range of sound frequencies audible to the human ear, generally 20 to 20,000 Hz. A normal speaker cabinet has an electro magnetically driven speaker cone sealed to an opening in the wall of a sealed cabinet. This arrangement provides a drooping frequency response curve (e.g., 22 in the graph 20 of FIG. 1).
The graph 20 of FIG. 1 represents a comparison of sound level verses frequency (i.e., frequency response). The plot 22 shows the drooping response for a closed cabinet system. Over the years, in an effort to improve sound quality low, mid, and high range speakers have been placed in separate cabinets or compartments. Each of those separate cabinets or compartments could then be tuned by creating ports with or without tubes in them into the cabinet to improve the frequency response. At low frequencies, the use of open ports or open ports and tubes into the speaker cabinet becomes unmanageable because of the large amount of air mass that needs to be moved to provide adequate tuning. As an example, an ideal cabinet size to hear low frequencies might be larger than the room in which the listener was sitting.
In an effort to offset the effects of a rigid sealed cabinet and avoid the spatial requirements necessary when attempting to create ports or tube ports with speakers at low frequencies, passive radiators (generally configured like speakers, but without the electro mechanical driver) have been placed in a secondary opening of the walls of the speaker cavity to reduce the drop-off of the loudness at low frequencies. An example of the improvement in the frequency response when such a passive radiator is installed is shown as plot 24 in FIG. 1. An example of the improvement in the frequency response attributable to the installation of a passive radiator is shown by plot 26 in FIG. 2. Note that the drop in the frequency response curve at lower frequencies in plot 26 is very severe before the range of inaudible frequencies 28 is reached. In this configuration , AREA2, the area under the curve to the right of the peak above a minimum loudness level is larger than AREA1 which is the area under the curve to the left of the peak. This imbalance is indicative of the relative distortion that can be heard as the loudness of the passive radiator nosedives and falls below an audible loudness. The low frequency loudness and energy are not balanced with the high frequency loudness and energy. The area under the curves provide a measure of the imbalance.
Recent trends in the audio systems market have been leaning towards enhancing the bass or sub-woofer response of the audio reproduction systems, so that even if a sound is below the low limit of the range of audible sound, the sound level is high so that the listener, although he or she cannot "hear" the sound in their ears, they can "feel" the sound as parts of their body are hit by the low frequency waves. At low frequencies, a limitation of passive radiators has been that the low frequencies require large displacements of the moveable radiator elements. Such large displacements can exceed the available range of motion of moveable radiator elements. For example, in FIGS. 4, 5, and 6, a speaker spider 62 at its perimeter is attached to the back end of a speaker basket 50 while the spider's center edge (or core) it is attached to the back end of a speaker cone 58 or a diaphragm 68 to spider 72 connection element 74. In each pictured radiator, a central moveable element is suspended by a speaker "surround" (52, 70, 84) which acts as the flexible element between the stationary front of the speaker basket (50, 66, 80) and the speaker moveable element. Because the range of travel available from each spider (62, 72, 88) is less than the range of travel available from the surround (52, 70, 84), as the spider (62, 72, 88) reaches the limit of its travel and stops. The sudden stop in the movement of the spider due to its full extensions causes distortions in adjacent components as well as in the pressure gradients in the speaker chamber. These distortions can be heard as static and/or unnatural discontinuities in the sound. The ratio of the speaker basket back opening "B" (which supports the spider) to the speaker basket front opening "A" (which supports the surround) is approximately 0.5 (or 50%).
In the instance when a passive radiator constructed solely of a speaker cone is connected only as its peripheral rim to a annular support surface in the wall of a speaker, for example, as shown in the U.S. Pat. No. 4,207,963, to Klasco, a larger range of travel is available to accommodate large movable element displacements experienced at high volumes at low frequencies. However, the use of a surround around the perimeter of the top of the cone and the cone shape produces cone wobble which also distorts the sound. The object of the Klasco patent was to arrange active elements to reduce the wobble in the passive radiator.
In the instance where a lone speaker cone suspended in a cavity opening is used, the response of the passive radiator during low frequency cycles as the cone is forced outward and pulled inward can be non-linear as the flexible member (surround) holding the cone tends to have different non-linear force to displacement characteristics when being stretched outwardly as compared to when it is being stretched inwardly.
The limitations on travel as shown in the prior art described in FIGS. 4, 5 and 6 and the wobble of a passive radiator as discussed in the Klasco patent and such a configuration's non-linearity, highlight the shortcomings of the prior art passive radiators.
The spatial requirement of the prior art passive radiators is also a drawback. The prior art passive radiators are quite large and bulky and extend a large distance into any sealed cavity. This spatial requirement must be taken into account when designing features and companion speakers to fit into the sealed cavity.